Active silicon optical bench

ABSTRACT

An integrated photonic module includes a semiconductor substrate configured to serve as an optical bench. Alternating layers of insulating and conducting materials are deposited on the substrate and patterned so as to define electrical connections. An optoelectronic chip is mounted on the substrate in contact with the electrical connections. A drive chip is mounted on the substrate so as to provide an electrical drive current to the optoelectronic chip via the electrical connections.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/473,659, filed Mar. 30, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/607,089, filed Jan. 28, 2015 (now U.S. Pat. No.9,647,419, issued May 9, 2017), which claims the benefit of U.S.Provisional Patent Application 61/980,055, filed Apr. 16, 2014, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to optoelectronic components andsystems.

BACKGROUND

Integrated photonic modules commonly comprise an optoelectronic chip,such as a laser diode chip, and ancillary micro-optical and/orelectronic components. The chip and ancillary components are mountedtogether on a bare silicon die, which is referred to as a “siliconoptical bench” (SiOB). In general, the components are aligned andcemented in place on the SiOB, and the optoelectronic chip iselectrically connected to the other electronic components by wirebonding. Alternatively, the optoelectronic and electronic components maybe mounted on a suitable printed circuit board and electricallyconnected by printed circuit traces.

As an example of this sort of construction, U.S. Patent ApplicationPublication 2011/0049334 describes an optical module, which transmitsoptical signals through a plurality of optical fibers in parallel. Theoptical module includes a substrate including an electrode pattern, aplurality of optical elements mounted on the electrode pattern of thesubstrate, and an electronic device mounted on the electrode pattern ofthe substrate and electrically connected to the optical elements. Theoptical elements and the electronic device are arranged on the substrateclose to each other such that lengths of the transmission lines betweenthe optical elements and the electronic device are minimized.

As another example, U.S. Pat. No. 7,496,251 describes apparatus andmethods for packaging optical communication devices include opticalbench structures, such as silicon-optical benches (SiOB). The opticalbench comprises a substrate having an electrical turning via formedtherein. An optoelectronic (OE) chip and integrated circuit (IC) chipare mounted on the optical bench and electrically connected using theelectrical turning via. The electrical turning via extends in directionsboth perpendicular and transverse to a surface of the substrate suchthat the OE chip and IC chip can be mounted on perpendicular surfaces ofthe optical bench in close proximity and electrically connected usingthe electrical turning via.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide novel photonic modules and methods for their production.

There is therefore provided, in accordance with an embodiment of theinvention, an integrated photonic module, which includes a semiconductorsubstrate configured to serve as an optical bench. Alternating layers ofinsulating and conducting materials are deposited on the substrate andpatterned so as to define electrical connections. An optoelectronic chipis mounted on the substrate in contact with the electrical connections.A drive chip is mounted on the substrate so as to provide an electricaldrive current to the optoelectronic chip via the electrical connections.

In some embodiments, the alternating layers of the insulating andconducting materials are patterned so as to define a transmission linebetween the drive chip and the optoelectronic chip, wherein thetransmission line has a characteristic impedance that matches an inputimpedance of the optoelectronic chip. Typically, the characteristicimpedance of the transmission line is less than 5Ω. In a disclosedembodiment, the layers of the conducting materials include a first metallayer configured as a ground plane and a second metal layer, separatedfrom the first metal layer by a layer of the insulating material, inwhich at least one feed line is formed, thereby defining thetransmission line. A ratio of a thickness h of the layer of theinsulating material to a width W of the at least one feed line istypically less than 1/50.

Additionally or alternatively, the transmission line includesdifferential transmission line, including a pair of parallel feed linesformed in one of the layers of the conducting material.

Further additionally or alternatively, the module includes a decouplingcapacitor, which is mounted on the substrate and interposed in thetransmission line between the drive chip and the optoelectronic chip.

In a disclosed embodiment, the optoelectronic chip includes a laserdiode, and the module includes at least one optical element mounted onthe substrate in alignment with the optoelectronic chip.

There is also provided, in accordance with an embodiment of theinvention, a method for producing a photonic module. The method includesproviding a semiconductor substrate configured to serve as an opticalbench, depositing alternating layers of insulating and conductingmaterials on the substrate, and patterning the layers so as to defineelectrical connections. An optoelectronic chip is mounted on thesubstrate in contact with the electrical connections, and a drive chipis mounted on the substrate so as to provide an electrical drive currentto the optoelectronic chip via the electrical connections.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a schematic, sectional illustration of an integrated photonicmodule (IPM), in accordance with an embodiment of the present invention;

FIG. 2 is a schematic top view of an IPM, in accordance with anembodiment of the present invention;

FIG. 3 is a schematic sectional illustration of an IPM, showingparameters and dimensions used in design of a transmission line in theIPM, in accordance with an embodiment of the present invention; and

FIG. 4. is an electrical circuit diagram of an IPM, in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Laser diodes are generally characterized by very low input impedance (onthe order of 1Ω) and commonly operate at high frequency. To drive alaser diode efficiently, it is therefore desirable that the wires ortraces connecting the laser to other circuit components have very lowinductance and low overall impedance. These requirements place tightconstraints on the design of the module containing the laser diode andgenerally dictate that the circuit components and the laser diode beplaced very close together in order to minimize the length (and hencethe inductance and overall impedance) of the conductors.

Embodiments of the present invention that are described herein providean active SiOB that mitigates these limitations. The SiOB is “active” inthe sense that it comprises multiple metal conducting layers, which aredeposited on the wafer that serves as the SiOB. The metal layers areseparated by one or more intervening insulating layers, such as oxide(SiO₂) layers, which are likewise deposited on the wafer. The metal andoxide layers may be deposited on the wafer and patterned using standardfabrication techniques.

Such fabrication techniques are capable of creating very thin insulatinglayers, on the order of 1 μm or less (in contrast to the much thickerinsulating layers in laminated and ceramic printed circuit boards). As aresult of the very thin insulating layers and the small distancesbetween the conductors in the metal layers that are deposited on thesilicon substrate, it is possible to create transmission lines on theSiOB of very low impedance—typically less than 5Ω, and even 1Ω or lesswhen needed—thus, matching the input impedance of devices such as alaser diode. The active SiOB thus serves two purposes at once:

-   -   It provides a precise, stable mounting platform for the optical        components of an integrated photonic module, such as        micromirrors and lenses, along with the laser diode itself; and    -   It provides well-matched, low-impedance electrical connections        between the electronic components of the module, including the        laser diode and the driver circuit.        Although the embodiments described hereinbelow relate mainly to        mounting of a laser diode and ancillary components on an SiOB,        the principles of the present invention may similarly be applied        using other types of semiconductor substrates and other sorts of        optoelectronic integrated circuits, including not only        transmitters, but also components such as modulators and        receivers.

Thus, some embodiments of the present invention provide an integratedphotonic module, which comprises a semiconductor substrate configured toserve as an optical bench. Alternating layers of insulating andconducting materials are deposited on the substrate and patterned so asto define electrical connections. An optoelectronic chip, such as alaser diode, is mounted on the substrate in contact with the electricalconnections. A drive chip is also mounted on the substrate so as toprovide an electrical drive current to the optoelectronic chip via theelectrical connections. The alternating layers of the insulating andconducting materials may be advantageously patterned so as to define atransmission line between the drive chip and the optoelectronic chip,with a characteristic impedance that matches the input impedance of theoptoelectronic chip.

FIG. 1 a schematic, sectional illustration of an integrated photonicmodule (IPM) 20, in accordance with an embodiment of the presentinvention. Beginning from the bottom up, IPM 20 comprises a siliconsubstrate 22, configured to serve as the SiOB of the IPM. Alternatinglayers of insulating material 26, 30, such as SiO₂, and conductivematerial 24, 28, such as gold, aluminum or another metal, are depositedon substrate 22. Although FIG. 1 shows only two insulating and twoconductive layers, in alternative embodiments (not shown in the figures)larger numbers of layers may be used in order to implement more complexcircuit designs.

Layers 24, 26, 28 and 30 are patterned, using photolithography or othertechniques that are known in the art of integrated circuits, to defineelectrical connections, as illustrated in the figures that follow. Theseconnections typically include conductive lines, vias, connection pads,and other structures as required. For example, first patterned metallayer 24 may be configured to serve as a ground plane, while secondpatterned metal layer 28 contains conductive lines and pads forconnecting an optoelectronic (OE) element 32, such as a laser diodechip, to a driver chip 34.

The components of IPM 20 are mounted over upper metal layer 28. Thesecomponents may comprise, for example, an optoelectronic element 32, suchas a laser diode chip, as well as a driver chip 34 and passivecomponents 36. These passive components may include both opticalcomponents and discrete electrical components, such as resistors andcapacitors. Although the electrical components within IPM are typicallyinterconnected by conductors in metal layers 24 and 28 deposited on theSiOB, wire bonding may be used to create additional electricalconnections, particularly between the IPM and external power, sensingand control components. Optical components, such as microlenses andmicromirrors, are mounted on the SiOB in precise alignment with thelaser diode or other OE elements. Optionally, the laser diode and otheroptical components may be protected by an additional optical element 38,such as a cover glass cap, which is supported over the SiOB by apatterned spacer 40. Additionally or alternatively, other sorts ofoptical components, such as lenses, optical patterning elements, and/oroptical filters, may be supported over the SiOB in this manner.

Reference is now made to FIGS. 2-4, which schematically illustrate anIPM 42 based on the layer structure of FIG. 1, in accordance with anembodiment of the present invention. FIG. 2 is a top view of IPM 42,while FIG. 3 is a partial sectional view, and FIG. 4 is an electricalcircuit diagram.

A laser diode 48 is mounted on an active SiOB 44 in IPM 42. Laser diode48 is connected to a current source 50 in a driver chip 52 by alow-impedance differential transmission line 56, which defines a currentloop, with a decoupling capacitor 54 interposed in the transmission lineon the ground (return) side. Transmission line 56 comprises a pair ofparallel feed lines 58, 60 in the upper metal layer (corresponding tolayer 28 in FIG. 1) on SiOB 44, with a ground plane 46 in the lowermetal layer (layer in FIG. 1). The current loop and ground plane areconnected respectively to a power supply pin (VCC) 62 and a ground pin64 of IPM 42.

Although feed lines 58 and 60 are shown in the figure as beingrelatively far apart, in actual designs the feed lines may be closelyspaced in order to minimize inductance. The electrical length of inputline 58 between driver 52 and laser diode 48 is equal to the electricallength of return line 60 through the capacitor.

Proper choice of design parameters of the transmission line, asdescribed further hereinbelow, makes it possible to match the impedanceof transmission line 56 to the input impedance of laser diode 48, whilestill permitting driver chip 52 and laser diode 48 to be placed farapart, typically up to about 8 mm apart (limited by attenuationeffects), relative to designs that are known in the art. This feature ofthe present design leaves more space for decoupling capacitor 54 andaffords greater flexibility in placement of optical components of IPM 42on SiOB 44.

FIG. 3 shows the parameters and dimensions used in determining thecharacteristic impedance of transmission line 56 in IPM 42. Theimpedance depends on the effective dielectric constant ε_(eff) of aninsulating layer 66 between feed lines 58, 60 and ground plane 46, whichin turn depends on the characteristic dielectric constant ε_(r) of theinsulating material making up layer 66 and the geometry of the feedlines and the insulating layer. Specifically:

$ɛ_{eff} = {\frac{ɛ_{r} + 1.0}{2} + {\frac{ɛ_{r} - 1.0}{2}\left\lbrack \frac{1}{\sqrt{1 + \frac{12h}{W}}} \right\rbrack}}$wherein h is the thickness of insulating layer 66, and W is the width offeed lines 58, 60, as illustrated in FIG. 3. For SiO₂, ε_(r)=10.

For a stripline-type transmission line (having only a single feed lineover the ground plane), the characteristic impedance is Z₀, as given bythe following formula (in which t is the thickness of the feed line):

$Z_{0} = {\frac{120\pi}{2.0\sqrt{2.0}\pi\sqrt{ɛ_{r} + 1.0}}\ln\left\{ {1.0 + {\frac{4.0h}{W^{\prime}}\left\lbrack {{\frac{4.0h}{W^{\prime}}\frac{14.0 + \frac{8.0}{ɛ_{eff}}}{11.00}} + \sqrt{{\left( \frac{14.0 + \frac{8.0}{ɛ_{eff}}}{11.0} \right)^{2}\left( \frac{4.0h}{W^{\prime}} \right)^{2}} + \frac{1.0 + \frac{1.0}{ɛ_{eff}}}{2.0}}} \right\rbrack}} \right\}}$In the above formula, W′=W+ΔW′,

${\Delta\; W^{\prime}} = {\Delta\;{W\left( \frac{1.0 + \frac{1.0}{ɛ_{eff}}}{2.0} \right)}\mspace{14mu}{and}}$${\Delta\; W} = {\frac{t}{}{\ln\left\lbrack \frac{4e}{\left( \frac{t}{h} \right)^{2} + {\left( \frac{1}{\pi} \right)^{2}\left( \frac{1}{{W\text{/}t} + 1.1} \right)^{2}}} \right\rbrack}}$

For a differential transmission line, such as transmission line 56comprising feed lines 58 and 60, the characteristic impedance is givenby:

$Z_{diff} = {2{Z_{0}\left\lbrack {1 - {0.48\exp\left\{ {{- 0.96}\frac{S}{h}} \right\}}} \right\rbrack}}$wherein S is the separation between the two feed lines, as shown in FIG.3. To achieve low impedance, on the order of a few ohms, with ε_(r)=10,the ratio of the thickness h of insulating layer 66 to the width W offeed lines 58 and 60 should be less than about 1/50. To reduce theimpedance to about 1Ω, the values should be such that h/W≈1/150. When his very small, for example 0.5 μm, as can be produced in the active SiOBdesign described above, the desired impedance can be achieved with W=75μm.

Although the figures and description presented above relate, for thesake of concreteness, to a certain specific IPM design, having a certainsequence of layers and layout of components and conductors within thoselayers, other designs based on the principles of the present inventionwill be apparent to those skilled in the art and are considered to bewithin the scope of the present invention. For example, although thefigures show an IPM design comprising two insulation layers and twometal layers formed on the silicon substrate, other embodiments may useonly a single metal layer (with or without an intervening insulationlayer) or three or more layers. Furthermore, such designs are notlimited to silicon substrates and may, alternatively, be implemented inmodules based on semiconductor substrates of other types.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. An integrated photonic module, comprising:a silicon substrate; alternating layers of insulating and conductingmaterials deposited on the substrate and patterned so as to defineelectrical connections; a laser diode on the silicon substrate incontact with the electrical connections; a drive circuit on the siliconsubstrate, coupled to provide an electrical drive current to the laserdiode via the electrical connections; and at least one optical elementon the silicon substrate in alignment with the laser diode, wherein thealternating layers of the insulating and conducting materials arepatterned so as to define a transmission line between the drive circuitand the laser diode, wherein the transmission line has a characteristicimpedance that matches an input impedance of the laser diode.
 2. Themodule according to claim 1, wherein the characteristic impedance of thetransmission line is less than 5 Ω.
 3. The module according to claim 1,wherein the layers of the conducting materials comprise a first metallayer configured as a ground plane and a second metal layer, separatedfrom the first metal layer by a layer of the insulating material, inwhich at least one feed line is formed, thereby defining thetransmission line.
 4. The module according to claim 3, wherein a ratioof a thickness h of the layer of the insulating material to a width W ofthe at least one feed line is less than 1/50.
 5. The module according toclaim 1, wherein the transmission line comprises differentialtransmission line, comprising a pair of parallel feed lines formed inone of the layers of the conducting material.
 6. The module according toclaim 1, and comprising a decoupling capacitor, which is mounted on thesubstrate and interposed in the transmission line between the drive chipand the optoelectronic chip.
 7. A method for producing a photonicmodule, the method comprising: providing a silicon substrate configuredto serve as an optical bench; depositing alternating layers ofinsulating and conducting materials on the substrate, and patterning thelayers so as to define electrical connections; placing a laser diode onthe substrate in contact with the electrical connections; providing anelectrical drive current to the laser diode via the electricalconnections; and mounting and aligning at least one optical element onthe silicon substrate with the laser diode, wherein depositing thealternating layers of the insulating and conducting materials comprisespatterning the alternating layers so as to define a transmission linebetween a drive chip and the laser diode, wherein the transmission linehas a characteristic impedance that matches an input impedance of theoptoelectronic chip.
 8. The method according to claim 7, wherein thecharacteristic impedance of the transmission line is less than 5 Ω. 9.The method according to claim 7, wherein patterning the alternatinglayers comprises configuring a first metal layer as a ground plane andforming at least one feed line in a second metal layer, separated fromthe first metal layer by a layer of the insulating material, so as todefine the transmission line.
 10. The method according to claim 9,wherein the alternating layers are deposited and patterned such that aratio of a thickness h of the layer of the insulating material to awidth W of the at least one feed line is less than 1/50.
 11. The methodaccording to claim 7, wherein patterning the alternating layerscomprises forming a differential transmission line comprising a pair ofparallel feed lines in one of the layers of the conducting material. 12.The method according to claim 7, and comprising interposing a decouplingcapacitor, mounted on the substrate, in the transmission line betweenthe drive chip and the optoelectronic chip.